We use the same color on analog wiring that we use on DC discrete wiring. In our case, that's blue for both leads. It makes sense, since the analog signals are usually powered from 24 VDC wires. We rely on the wire numbers to differentiate between + and - signals.

I've seen lots of attempts to use a code, say red or white for +, black for - . The problem is that if you have multiple devices in the loop, like an indicator, an analog card, and a recorder, you have to go "+ to +" or "- to -" between devices at some point. This interferes with the consistency of the color code. If you have a loop powered device you start out with the power supply +24 VDC and this is blue anyway. I gave up trying to fight it, and went to our present scheme.

We use standard DC wiring.
Black is neg or common or chassis ground.
Green or bare is power supply ground, bonding conductor, or shield.
Everything else is positive. Many cables use a combination of colours, like orange-blue, white-red, yellow-white. Some twist pairs
just to confuse you. On these you'll have to make up your own rules.
Tom is quite right. A loop can go through a meter, Input module, amp or transducer, good luck matching colours
Have fun!
Bruce

Like Tom said, this is a tough question, with the way that analog loops work.

However, we typically use two wire belden cable with a clear and black lead plus a bare shield. For basic loops, we generally use black for the negative and clear for the positive. Complex loops usually start that way, but in the end you end up having to cross things up. Your best bet is to rely on the wire numbers to keep things straight.

Thermocouple wires are a whole different animal. No one has mentioned them yet, but I feel like I should at least throw them in the discussion. Each type of thermocouple has its own associated wire type. The wire type is generally identified by the wire colors and labeling on the outside of the cable. There are typically two wires, one red and one another color which varies depending on the type of thermocouple that the wire is intended for. The red wire is always the Negative (which still strikes me as odd, but that's the way it is) AFAIK, these are always wired point-to-point. One end being the thermocouple itself, and one end being the controller/measuring device. Terminals can be used, but these need to be special terminals designed for the type of thermocouple to be used.